Dell SK 8115 keyboard settings

I am using this Dell keyboard with Windows 7 Pro x64, Firefox, and Thunderbird, all of which are up-to-date. A few days ago, the UP Arrow and DOWN arrow vertical scroll function changed from five lines to several pages with just one click of either arrow key. How can I reset this function to scroll just five lines, as it used to do, instead of several pages? I can't relate this change to any system or app changes that I might have made, and system restore didn't solve the problem. Thank you for your cooperation and assistance. Please reply to <ADMIN NOTE: Email id removed per privacy policy>